This invention relates generally to furnaces and, more particularly, to a method and apparatus for controlling the operation of the furnace so as to match its output with the load.
In conventional gas furnaces, it is generally required to include a limit control to protect against excessively high supply air temperatures as the airflow is decreased. Because of its ability to be modulated, the electronic furnace described in U.S. Pat. No. 4,638,942 issued on Jan. 27, 1987 and assigned to the assignee of the present invention is better adaptable to zone control air distribution systems. However, even in this system the combustion can only be modulated down to 66 percent of its full rate, since operating at any lower rate would create primary heat exchanger cold spot problems and would require improved burner turn-down capability. Also, it operates either at high or low input and is not capable of operating therebetween. Further, it requires a complex software algorithm for implementation of the combustion modulation.
In accordance with another embodiment of the above-mentioned electronic furnace, an electronically commutated motor is applied to drive the circulating air blower at variable speeds to obtain a constant temperature rise for all steady state operating conditions. Such a system is described in U.S. Pat. No. 4,688,547 issued on Aug. 25, 1987 and assigned to the assignee of the present invention. However, it should be recognized that in this system a limit switch is required to guard against excessive heat exchanger and supply air temperatures, such as when the blower operates against extremely high static pressures, and is unable to maintain a fixed airflow.
In a zone controlled system, it is necessary to have a blower that is sized to provide a minimum amount of air to the system when all of the dampers are open. When the set points of certain areas are met, the associated dampers are closed and, in order to accommodate the increased static pressure in the remaining ducts, it is generally necessary to provide a bypass duct. The need for such a duct is not only cause for added expense, but also tends to decrease the efficiency of the system.
Fixed-rate integrated space heat/domestic hot water heating systems are now becoming relatively common. Such a system is shown in U.S. Pat. No. 4,727,727 issued on Mar. 1, 1988. The principle is to circulate stored potable hot water to the fan coil, over which a fixed speed blower circulates air to be warmed and distributed to the spaces. One of the advantages of such a system is that the supply air delivered to the space to be heated cannot be hotter than the water circulated from the storage tank. Thus, as circulating airflow decreases, supply air temperature will approach an upper limit equal to the storage tank water temperature.
Despite the advantage of the inherent temperature limiting feature of such a system, the storage tank, water heater, pump, fan coil, and circulating air blower are typically sized for a fixed heat output. For instance, a system designed to provide 30,000 BTUH space heating and 800 CFM for a two-ton air conditioner would have components sized accordingly. Thus, when a call for heat is made, the system has relatively high output until that demand is met and then it is turned off and remains off until the temperature drops to a predetermined lower level at which time the system is then turned on to the relatively high output again. Such a system is thus relatively noisy since it operates only at a relatively high speed. Further, the level of comfort provided is somewhat minimized by the temperature swing that is produced by fixed output heating appliances when cycled on/off at full capacity.
It is therefore an object of the present invention to provide an improved heating system which does not require a limit control to protect against excessive heat exchanger thermal stress and high supply air temperatures when the airflow volume decreases.
Another object of the present invention is the provision of an improved heating system that is adaptable to zone controlled air distribution systems but does not require a bypass duct.
A further object of the present invention is an improved heating system that does not require a complex modulating combustion system.
Still another object of the present invention is the provision for an improved heating system wherein heat exchanger cold spot corrosion is avoided.
Yet another object of the present invention is the provision for an improved heating system which is quiet in operation while maintaining the temperature of a space to be heated within a relatively narrow temperature range.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.